(1) Field of the Invention
The present invention generally relates to a method for correcting an image signal deteriorated by flare and an image reading unit, and more particularly to a method for correcting an image signal deteriorated by flare and an image reading unit in which a high quality image can be obtained. The method for correcting an image signal and the image reading unit according to the present invention can be applied to copy machines, optical scanners and facsimile machines.
(2) Description of Related Art
Flare can be generated in an image reading unit, as shown in FIGS. 1A and 1B. FIG. 1A is a vertical cross sectional view of a mechanism in which a document is irradiated by a light source, and FIG. 1B is a transverse cross sectional view thereof. Referring to FIGS. 1A and 1B, a document 33 is placed on a contact glass 34. A light source 31 is provided between a frame 32 and the contact glass 34. A fluorescent lamp extending in a main scanning direction is, for example, used as the light source 31. The document 33 includes a black part 33a corresponding to a visual image and a white part 33b corresponding to a background of the document 33. The light source 31 emits light rays in any directions, so that some parts of the light rays, which are reflected by the surface of the document 33, are reflected again by the frame 32 and the surface of the light source 31. Thus the light rays which are reflected by the frame 32 and the surface of the light source 31 can be reprojected onto the surface of the document 33. Light rays which are reflected a plurality of times between the frame 32 and the document 33 placed on the contact glass 34, as described above, are often referred to as flare. That is, flare refers to light rays incident via optical paths other than a primary path to the document 34. When image signals detected at an area are affected by the flare caused by reflected light rays on a surrounding area thereof, a MTF (Modulation Transfer Function) of an image reproduced based on the image signals deteriorates. Thus, in this case, a high quality image can not be obtained. The MTF can describe a degree of sharpness of an image.
Flare affects the image signal as in the following manner.
When a document having a white solid image is scanned on a main scanning line A-B, as shown in FIG. 2A, an image signal as shown in FIG. 2B is obtained by the image reading unit. On the other hand, when a document having a white line formed on a black background is scanned on the main scanning line A-B, as shown in FIG. 2C, an image signal as shown in FIG. 2D is obtained. Even if a density of the white solid image in a case shown in FIG. 2A is equal to that of the white line in a case shown in FIG. 2C, the level of the image signal obtained as shown in FIG. 2B is greater by .epsilon. than the level of the image signal corresponding to the white line obtained as shown in FIG. 2D. The amount of flare caused by the black background surrounding the white line (see FIG. 2B) is less than the amount of flare caused by the white image because a reflectivity of the black background is less than that of the white image. Thus, the above difference between the image signals shown in FIGS. 2B and 2D is generated.
Conventionally, to decrease the amount of flare affecting the image signal as has been described above, a light source, for example, disclosed in Japanese Patent Laid Open Publication No. 1-182864 has been proposed, the light source being used in an image reading unit.
In a case where a light source 44 having a lamp tube 42, such as a fluorescent lamp tube, and a reflector 43, is placed under a support glass 41, light rays emitted from the light source 44 are diffused in any direction as shown in FIG. 3. According to the prior art disclosed in Japanese Patent Laid Open Publication No. 1-182864, the amount of flare is decreased by decreasing a degree of diffusion of the light rays emitted from the light source, as shown in FIGS. 4A and 4B.
Referring to FIGS. 4A and 4B, a light source 53 has a lamp tube 48 and a reflector 49. Each side end of the reflector 49 is provided with plates 50 and the inside of the reflector 49 is partitioned into plurality of areas by partition plates 51. Due to the above structure of the light source 53, some of the light rays diffused from the lamp tube 48 are interrupted by the partition plates 51. That is, some of the light rays are reflected by surfaces 51a and 51b of each of the partition plates 51, so that the light rays reflected thereby do not reach a document placed on a support glass 52. As a result, the degree of diffusion of the light rays emitted from the light source 53 is decreased as shown in FIG. 4A.
When the degree of diffusion of the light rays emitted from the light source is decreased, the amount of flare applied to each point on the document is also decreased. That is, the amount of flare affecting the image signal obtained at each point on the document is reduced to a small amount.
In the above prior art, to definitely decrease the flare, the number of partition plates 51 must be increased and an end of each of the partition plates 51 must be brought close to the support glass 52. However, when the number of partition plates 51 is increased and the end of each of the partition plates 51 is brought close to the support glass 52, the surface of the document can not be uniformly irradiated by the light rays. In addition, the amount of light projected onto the surface of the document is also decreased. As a result, a high quality image cannot be obtained.